Pfizer has increased the pressure on AstraZeneca by sidestepping the UK pharmaceutical giant's board and making a direct appeal to shareholders that it should be allowed to buy the company.

The move came ahead of appearances by the bosses of Pfizer and AstraZeneca at business select committee on Tuesday morning, where MPs will challenge the US company over its commitment to UK jobs.

In a statement to the City, Pfizer said it "remains disappointed at the lack of engagement by the AstraZeneca board" since its initial approach, back in January.

The New York-headquartered company said it believed there was "a compelling rationale for a combination" and argued AstraZeneca would face "challenges" if it remains independent.

These challenges were listed as declining revenues as lucrative drugs lose patent protection, uncertainty over whether AstraZeneca's research and development will produce viable drugs, and a "potential lack of scale" to compete against larger rivals.

Adrian Bailey, the Labour MP, who chairs the business, innovation and skills committee, told BBC Radio 4's Today programme that Pfizer had a reputation for buying companies with good research facilities and then closing them down. "Pfizer's track record gives no confidence that they can deliver on the promises they have made."

His warning was echoed by the Royal Society, which weighed into the debate with a letter to MPs raising concerns that Pfizer's commitments are vague and should be treated with caution.

Professor Sir Paul Nurse, the president of the Royal Society and a Nobel Prize-winning geneticist, told MPs that Pfizer's promises, which run for five years, are insufficient. "A five-year commitment to the UK is insufficient. A commitment of at least 10 years is required. Science is not a quick-win sector – it requires long-term investment."

In a letter to MPs, seen by the BBC, Sir Paul said the firm should promise £100m in funds for collaboration with universities and small and medium-size businesses.

He said that private investment in the sector in the UK was "woefully low" compared with rivals such as the US and Germany and that the merger had the potential to be a force for good.

But he added: "Any potential threat to the long-term strength of our life sciences sector, and thus to our national economic interest, must be taken very seriously ."